1. Field of the Invention
The present invention relates generally to the production of low weight, high strength gypsum wallboard and gypsum core compositions. More specifically, the invention is directed to a method of providing void spaces in gypsum wallboard and in a gypsum core composition.
2. Description of Related Technology
A common method of constructing walls and ceilings includes the use of inorganic wallboard panels or sheets, such as gypsum wallboard, often referred to simply as xe2x80x9cwallboardxe2x80x9d or xe2x80x9cdrywall.xe2x80x9d Wallboard can be formulated for interior, exterior, and wet applications. The use of wallboard, as opposed to conventional boards made from wet plaster methods, is desirable because the installation of wallboard is ordinarily less costly and less cumbersome when compared to the installation of conventional plaster walls.
A major ingredient of the gypsum wallboard core (hereinafter xe2x80x9cwallboard corexe2x80x9d or xe2x80x9ccorexe2x80x9d) is calcium sulfate hemihydrate, commonly referred to as xe2x80x9ccalcined gypsum,xe2x80x9d xe2x80x9cstucco,xe2x80x9d or xe2x80x9cplaster of Paris.xe2x80x9d Stucco has a number of desirable physical properties including, but not limited to, its fire resistance, thermal and hydrometric dimensional stability, compressive strength, and neutral pH. Typically, stucco is prepared by drying, grinding, and calcining natural gypsum rock (i.e., calcium sulfate dihydrate). The drying step in the manufacture of stucco includes passing crude gypsum rock through a rotary kiln to remove any free moisture present in the rock. The dried rock is then passed through a roller mill (a type of pulverizer), wherein the rock is ground or comminuted to a desired fineness. The degree of comminution is determined by the ultimate use. The dried, fine-ground gypsum can be referred to as xe2x80x9cland plaster,xe2x80x9d regardless of its intended use.
The land plaster is used as feed in calcination processes for conversion to stucco. The calcination step in the manufacture of stucco is performed by heating the land plaster to liberate a portion of the chemically bound water molecules. The calcination of stucco can generally be described by the following chemical equation which shows that heating calcium sulfate dihydrate yields calcium sulfate hemihydrate (stucco) and water vapor:
CaSO4.2H2O+heatxe2x86x92CaSO4.xc2xdH2O+1xc2xdH2O. 
This calcination process step is performed in a xe2x80x9ccalciner,xe2x80x9d of which there are several types known by those of skill in the art.
Uncalcined calcium sulfate (i.e., land plaster) is the xe2x80x9cstablexe2x80x9d form of gypsum. However, calcined gypsum, or stucco, has the desirable property of being chemically reactive with water, and will xe2x80x9csetxe2x80x9d rather quickly when the two are mixed together. The setting reaction is a reversal of the above-described chemical reaction that occurs during the calcination step, and is generally described by the following chemical equation showing that calcium sulfate hemihydrate is rehydrated to its dihydrate state:
CaSO4.xc2xdH2O+1xc2xdH2Oxe2x86x92CaSO4.2H2O+heat. 
The actual time required to complete the setting reaction generally depends upon the type of calciner and the type of gypsum rock that is used to produce the gypsum, and can be controlled within certain limits by the use of additives such as, for example, retarders, set accelerators, and/or stabilizers. The time required for rehydration can be as little as about two minutes to as long as about eight hours depending on the quantity of retarders, set accelerators, and/or stabilizers present.
Generally, wallboard is produced by enclosing a core of an aqueous slurry of calcined gypsum and other materials between two large sheets of board cover paper. Methods for the production of gypsum wallboard generally are described, for example, by Michelsen, T. xe2x80x9cBuilding Materials (Survey),xe2x80x9d Kirk-Othmer Encyclopedia of Chemical Technology, (1992 4th ed.), vol. 4, pp. 618-619, the disclosure of which is hereby incorporated herein by reference.
A conventional process for manufacturing the core composition of gypsum wallboard initially includes the premixing of dry ingredients in a high-speed mixing apparatus. The dry ingredients can include calcium sulfate hemihydrate (stucco), an accelerator, and an antidesiccant (e.g., starch). The dry ingredients are mixed together with a xe2x80x9cwetxe2x80x9d (aqueous) portion of the core composition in a pin mixer apparatus. The wet portion can include a first component, commonly referred to as a xe2x80x9cpaper pulp solution,xe2x80x9d that includes a mixture of water, paper pulp, and, optionally, one or more fluidity-increasing agents, and set retarders. The paper pulp solution provides a major portion of the water that forms the gypsum slurry of the core composition. A second wet component can include a mixture of strengthening agents, foaming agents, and other conventional additives, if desired. Together, the aforementioned dry and wet portions comprise an aqueous gypsum slurry that eventually forms the gypsum wallboard core.
In the production of exterior sheathing and moisture-resistant wallboard cores, various materials, such as silicone water repellents, waxes, and asphalt emulsions, can be incorporated into the gypsum wallboard core to impart increased water absorption resistance to the board. These materials are typically supplied as water emulsions to facilitate ease of incorporation into the board core, and can be added directly into the mixing apparatus or incorporated into the pulp solution prior to addition to the mixing apparatus.
After the aqueous gypsum slurry is prepared, the slurry and other desired ingredients are continuously deposited to form a gypsum wallboard core slurry between two continuously-supplied moving sheets of cover paper. Various types of cover paper (or xe2x80x9ccover sheetsxe2x80x9d) are known in the art. The two cover sheets typically comprise a pre-folded face paper and a backing paper. As the slurry is deposited onto the face paper, the backing paper is brought down atop the deposited core slurry and bonded to the prefolded edges of the face paper. The core slurry is then allowed to cure or set (i.e., react with the water present in the aqueous slurry), whereby calcium sulfate hemihydrate is converted to calcium sulfate dihydrate.
The setting reaction produces gypsum crystals which are interwoven. The resulting crystal-to-crystal interactions contribute to the strength of the wallboard core. The gypsum crystals also preferably interlock with paper fibers protruding from the surface of the cover sheets, thereby bonding the cover sheets to the wallboard core. This bonding-type interaction also increases the strength of the wallboard product.
After the core has set, the formed sheet is dried to remove any excess water, and the board is cut into standard sizes. Standardized sheets (or panels) of wallboard typically are about four feet (about 1.22 meters) wide and about 8 feet to about 16 feet (about 2.4 meters to about 4.9 meters) in length. Sheets typically are available in thicknesses varying in a range of about xc2xc inch to about one inch (about 0.6 centimeters to about 2.6 centimeters).
In order to provide satisfactory strength, commercially-available gypsum wallboard generally requires a density of about 1675 to 1700 pounds per thousand square feet (lbs/MSF) of one-half inch board. Because heavy or high-density gypsum wallboards are more costly and difficult to manufacture, transport, store, and manually install at job sites when compared with lighter or low-density boards, various attempts have been made to reduce board weight and density without sacrificing board strength. Often, however, where wallboard is formulated to have a density less than about 1675 to 1700 lbs/MSF of one-half inch board, the resulting strength is unacceptable for commercial sale.
It has been previously disclosed that reduced density wallboard can be obtained by mixing an aqueous foam into the gypsum slurry. The density of the wallboard is reduced because the foam introduces air voids into the gypsum wallboard core composition. However, if the foam substantially degrades during mixing of the gypsum slurry and/or while the gypsum core is setting, the gypsum slurry will fill those void spaces left by ruptured bubbles, and a reduced board weight will not be achieved. Furthermore, many of the lighter and less dense wallboard products obtained by incorporating foams into the gypsum slurry possess inferior mechanical properties which render them ill-suited for commercial use.
For example, U.S. Pat. No. 4,156,615 to Cukier, the disclosure of which is hereby incorporated by reference, discloses a foaming agent which can be used to introduce air voids into a gypsum wallboard. However, gypsum wallboard produced with this surfactant blend must be formulated at heavier board weights (approximately 1675 lbs/MSF) in order to meet ASTM nail pull standards. Further, the foaming agent of the ""615 patent produces more stable air bubbles upon foaming. Because a large number of stable air bubbles are introduced into the gypsum slurry, a large number of small voids are provided in the gypsum wallboard, and the mechanical properties of the produced gypsum board are deleteriously affected.
U.S. Pat. No. 5,240,639 to Diez et al., the disclosure of which is hereby incorporated by reference, also discloses a foaming agent which can be used to introduce air voids into a gypsum wallboard. Gypsum wallboard produced with this surfactant blend also must be formulated at heavier board weights (approximately 1675 lbs/MSF) in order to meet ASTM nail pull requirements. Further, the foaming agent of the ""639 patent produces less stable air bubbles which frequently coalesce and rupture during mixing of the gypsum slurry and/or while the gypsum slurry is setting. As a consequence, greater amounts of this foaming agent must be used to achieve significant weight reduction in gypsum wallboards.
U.S. Pat. No. 5,643,510 to Sucech, the disclosure of which is hereby incorporated by reference, discloses a process for producing xe2x80x9cfoamedxe2x80x9d gypsum board wherein a stable foaming agent comprising alkyl ether sulfates and an unstable foaming agent comprising alkyl sulfates are blended and introduced into a gypsum slurry. The foaming agents of the ""615 patent and of the ""639 patent are both disclosed to be stable foaming agents.
It therefore remains desirable to provide void spaces in a gypsum wallboard such that the board weight is reduced without deleteriously affecting the mechanical properties of the gypsum wallboard.
It is also desirable to provide void spaces in a gypsum wallboard wherein the air bubbles do not prematurely rupture thereby allowing the gypsum slurry to fill the void spaces left by the ruptured bubbles.
Furthermore, it would be desirable to produce high-strength gypsum wallboard having weights and densities generally less than those produced by conventional methods. Reduced weight and density boards, however, should meet industry standards and have strengths similar to, or greater than, conventional wallboard. Moreover, such wallboard also should be able to be manufactured using high-speed manufacturing apparatus and not suffer from other negative side effects. For example, such high-strength wallboard should be able to set and dry within a reasonable period of time.
The present invention provides a method of providing void spaces in a gypsum wallboard such that the board weight is reduced without deleteriously affecting the mechanical properties of the gypsum wallboard.
The invention also provides a method of providing void spaces in a gypsum wallboard wherein air bubbles do not prematurely rupture during mixing of the gypsum slurry and/or during the setting of the gypsum slurry, thereby allowing the gypsum slurry to fill the void spaces left by the ruptured bubbles.
The invention further provides a method of providing void spaces in a gypsum wallboard such that high-strength gypsum wallboard having weights and densities generally less than those produced by conventional methods is produced.
In accordance with one aspect of the present invention, a method of providing void spaces in a gypsum wallboard comprises a step of adding air bubbles to a gypsum slurry. The slurry typically comprises calcium sulfate hemihydrate and sufficient water to hydrate the calcium sulfate hemihydrate. Air bubbles can be formed by foaming a first foaming agent and a second foaming agent in water. The first and second foaming agents produce air bubbles that differ in stability. More specifically, the first foaming agent produces less stable air bubbles and the second foaming agent produces more stable air bubbles. The less stable air bubbles are sufficiently unstable such that they coalesce on contact with each other in the slurry to form larger air bubbles. The weight ratio of the less stable air bubbles to the more stable air bubbles initially added to the slurry is such that there are a sufficient number of more stable air bubbles to prevent a majority of the less stable air bubbles from becoming large enough to rupture before the slurry sets sufficiently, and to prevent the slurry from filling void spaces left by ruptured bubbles. The method further includes the step of mixing the air bubble-containing gypsum slurry to distribute the air bubbles throughout the slurry. The gypsum slurry forms a core composition of the gypsum wallboard. The core composition is deposited on a cover sheet, and, a second cover sheet is applied over the resulting core composition to form a gypsum wallboard.
In accordance with an additional aspect of the present invention, a method of providing void spaces in a gypsum core composition comprises the step of adding air bubbles to a gypsum slurry. The slurry typically comprises calcium sulfate hemihydrate and sufficient water to hydrate the calcium sulfate hemihydrate. Air bubbles can be formed by foaming a first foaming agent and a second foaming agent in water. The first and second foaming agents produce air bubbles that differ in stability. More specifically, the first foaming agent produces less stable air bubbles and the second foaming agent produces more stable air bubbles. The first foaming agent is described by the chemical formula R(OCH2CH2)aOSO3M1 wherein R represents linear and branched hydrocarbons having a maximum molecular weight of about 169, and mixtures thereof, a is the average number of moles of ethylene oxide per mole of foaming agent and is in the range of 0.4 and 1.3, and, M1 is selected from the group consisting of sodium, potassium, magnesium, ammonium, quaternary ammonium, and mixtures thereof. The second foaming agent is described by the chemical formula CH3(CH2)bCH2(OCH2CH2)cOSO3M2 wherein the average value of b is in the range of 6.5 and 7.5, c is the average number of moles of ethylene oxide per mole of foaming agent and is in the range of 1.5 and 2.5, and, M2 is selected from the group consisting of sodium, potassium, magnesium, ammonium, quaternary ammonium, and mixtures thereof. The weight ratio of the first foaming agent to the second foaming agent is in the range of 75:25 to 95:5. The presence of the more stable air bubbles in the slurry increases the residence time of the less stable bubbles in the slurry, thereby preventing the premature rupturing of the less stable bubbles before the gypsum slurry sets sufficiently, and thereby preventing the slurry from filling the void spaces formed by the ruptured air bubbles. The method further includes the steps of mixing the air bubble-containing gypsum slurry to distribute the air bubbles throughout the slurry, the gypsum slurry forming a core composition of the gypsum wallboard, depositing the core composition on a cover sheet, and applying a second cover sheet over the core composition to form a gypsum wallboard.
In accordance with yet another aspect of the present invention, a method of providing void spaces in a gypsum wallboard core composition comprises a step of adding air bubbles to a gypsum slurry. The slurry typically comprises calcium sulfate hemihydrate and sufficient water to hydrate the calcium sulfate hemihydrate. Air bubbles can be formed by foaming a first foaming agent and a second foaming agent in water. The first and second foaming agents produce air bubbles that differ in stability. More specifically, the first foaming agent produces less stable air bubbles and the second foaming agent producing more stable air bubbles. The less stable air bubbles are sufficiently unstable such that they coalesce on contact with each other in the slurry to form larger air bubbles. The weight ratio of the less stable air bubbles to the more stable air bubbles initially added to the slurry is such that there are a sufficient number of more stable air bubbles to prevent a majority of the less stable air bubbles from becoming large enough to rupture before the slurry sets sufficiently, and to prevent the slurry from filling void spaces left by ruptured bubbles. The method further includes the steps of mixing the air bubble-containing gypsum slurry to distribute the air bubbles throughout the slurry, and, depositing the gypsum slurry on a substrate.